Seals are used in aircraft engines to isolate a fluid from one or more areas/regions of the engine. For example, seals control various parameters (e.g., temperature, pressure) within the areas/regions of the engine and ensure proper/efficient engine operation and stability.
Referring to FIGS. 2A-2B, a prior art sealing system 200 is shown. The system 200 is used to provide an interface between a static engine structure 206 and a rotating engine structure 212. The system 200 includes a floating, non-contact seal 218 that is formed from beams 230a and 230b and a shoe 236 coupled to the beams 230a and 230b. The seal 218 may interface to the structure 206 via a carrier 242. A spacer 248 may separate the carrier 242 and/or the beams 230a and 230b from a seal cover 254. Secondary seals 260 may be included in a cavity formed between the spacer 248, the cover 254 and the shoe 236. The spacer 248 and/or the seal cover 254 may help to maintain an (axial) position of the secondary seals 260. The shoe 236 may interface to (e.g., may slide or rotate with respect to) a scalloped plate 266. The seal 218 may include at least some characteristics that are common with a HALO® seal provided by, e.g., Advanced Technologies Group, Inc. of Stuart, Fla.
In operation, air flows from a high pressure area/region 270 of the engine to a low pressure area/region 280 of the engine as shown via the arrow 284. As the air flows passes teeth 238 of the shoe 236 (where the teeth 238 are frequently formed as thin knife-edges), an associated pressure field changes. This change induces the shoe 236 to move in, e.g., the radial reference direction until an equilibrium condition is obtained. In this respect, the seal 218 is adaptive to changing parameters and allows for maintenance of clearances between the structures 206 and 212 within a relatively tight range in order to promote engine performance/efficiency. The secondary seals 260 may promote the flow 284 from the high pressure region 270 to the low pressure region 280 as shown between the shoe 236 (e.g., teeth 238) and the rotating structure 212.
As shown in FIG. 2B, the shoe 236 may be manufactured to include hooks 236a and 236b that may selectively mate with T-stops 288a and 288b, respectively, formed in an outer ring structure 288. For example, when the shoe 236 deflects radially outboard/outward, the hook 236b may contact the radially inner end of the T-stop 288b to stop further outward movement of the shoe 236. Similarly, when the shoe 236 deflects radially inboard/inward, the hook 236a may contact the radially outer end of the T-stop 288a to stop further inward movement of the shoe 236.
Referring to FIG. 2C, a cold build gap 292 may be defined between the radial inner end of the shoe 236 (e.g., the teeth 238) and a radial outer end of the structure 212. The cold build gap 292 may be used to account for build/component tolerances and may help to avoid contact between the shoe 236/teeth 238 and the structure 212 during assembly.
The reference character 294 reflects the potential distance that the shoe 236 may move/deflect radially outward during engine operation relative to the cold/non-operational state of the engine. Similarly, the reference character 296 reflects the potential distance that the shoe 236 may move/deflect radially inward during engine operation relative to the cold/non-operational state of the engine. The range of potential positions of the shoe 236, in conjunction with reference characters 294 and 296, is reflected as existing between the phantom/dashed lines 294a and 296a. While the range between the lines 294a and 296a is shown as being substantially uniform over the axial length of the shoe 236, one skilled in the art would appreciate that different portions of the shoe 236 in, e.g., the axial direction may deflect/move in different amounts. For purposes of this disclosure, such differences may largely be ignored as the movement may be analyzed/assessed relative to a reference point taken on the shoe 236.
As shown in FIG. 2C, relative to the cold/non-operational state of the engine, the potential outward deflection 294a may be greater than the potential inward deflection 296a (in terms of magnitude of deflection). In some embodiments, the opposite scenario may be true, e.g., the potential inward deflection may be greater than the potential outward deflection. Irrespective, to the extent that there is an imbalance between the outward and inward deflections that may be experienced by the shoe 236, this imbalance may result in a non-zero mean stress value imposed on the seal 218 (e.g., the beams 230a and 230b). This non-zero mean stress value may cause/increase material fatigue, thereby reducing the lifetime of the seal 218.